Microtubules are ubiquitous organelles in the cytoplasm of cells. The molecular mechanism by which they form from dimeric subunits of tubulin is not well understood. Calcium appears to play a critical role in the regulation of the assembly process, but the mechanism of its action remains in question. Recent studies in vitro polymerization of tubulin extracted from the brain of dogfish sharks (Langford, 1978, ECR 111:139) show that this tubulin exhibits a unique response to Ca ions. Millimolar concentrations of Ca ions cause it to form "coiled sheet polymers." Using dogfish braid as the source of tubulin, the objectives of the proposed research are to determine the effective Ca ions concentration range over which the "Ca ions effect" is observed, the pathway by which microtubules transform into coiled sheets of protofilaments when Ca ion is added, the specificity of Ca ions, and the change in the number of free sulfhydryls in the presence of Ca ions. The kinetics of the transformation of microtubules into coiled sheet polymers will be determined spectrophotometrically. The size and shape of the intermediates formed during the transformation will be determined by electron microscopy.